The software that controls wireless networking chipsets made by Realtek Semiconductor contains a critical vulnerability that could allow attackers to compromise home routers.

The flaw exists in a firmware component called miniigd that's present in router models based on Realtek chipsets. The component is part of the software development kit (SDK) for RTL81xxx chipsets that Realtek provides to router manufacturers.

"An attacker could leverage this vulnerability to execute code with root privileges," the ZDI team said in an advisory published Friday. Exploitation does not require authentication, it said.

The team claims that it tried to privately report the issue to Realtek on four different occasions since August 2014, but the company failed to respond so a decision was made to disclose the issue publicly.

It's hard to estimate how many devices are potentially affected. Lawshae said on Twitter that some routers made by TRENDnet and D-Link were confirmed to be vulnerable, but devices from other manufacturers are also likely to be affected.

A search on WikiDevi, an online database of computer hardware, revealed over 350 wireless networking devices with RTL81xxx chipsets. However, the version of the Realtek SDK used in those devices also comes into play and that's harder to determine.

One way to exploit the vulnerability is through the Universal Plug and Play (UPnP) service, which on some devices is accessible via the Internet in addition to the local area network. A search with SHODAN revealed around 480,000 routers that respond to UPnP requests over the Internet and include "Realtek/V1.3" in their response. Another 350,000 devices indicate older Realtek SDK versions.

It's worth noting that even if a device does not expose the UPnP service to the Internet, it is still vulnerable to attacks over the local network if it uses the vulnerable miniigd component. In addition, other services running on the router might also use miniigd, opening additional attack vectors.

Realtek did not immediately respond to a request for comment. However, even if the company is aware of the issue and provided SDK patches to manufacturers, it's likely that many affected devices will never be fixed because they're old and no longer supported.

In the absence of patches, users have few options to protect their devices. Possible mitigations include disabling UPnP and restricting which computers can communicate with the other services on a vulnerable device, including it's Web-based administration interface.

"This could be accomplished in a number of ways, most notably with firewall rules/whitelisting," the ZDI team said in their advisory. "These features are available in the native Windows Firewall."